1. Field of the Invention
This invention relates to transmission electron microscopes, and particularly to the auto-adjustment of high resolution transmission electron microscopes.
2. Description of Prior Art
Modem transmission electron microscopes (TEMS) are able to resolve individual atoms and columns of atoms in thin samples of inorganic and organic materials. However, the images produced by such microscopes can only be correctly interpreted in terms of the atomic structure of the sample if the imaging parameters of the microscope are controlled with a very high degree of accuracy. In particular, the focus of the objective lens of the TEM must be set with a precision better than about 2 nm, the astigmatism of the objective lens must be set with a precision better than about 1 nm, and the direction of the electron beam incident on the sample must be adjusted to be parallel to the optic axis of the microscope objective lens with a precision better than about 10.sup.-4 radians.
Making the needed adjustments is a task requiring a high level of theoretical knowledge and experimental skill, which restricts the availability of the atom-resolving ability of modern TEMs to a handful of experts. If the needed adjustments could be performed automatically, the power of modem TEMs would become available to a much wider range of scientific researchers.
A method for auto-adjusting a TEM is known from an article by W. O. Saxton, D. J. Smith and S. J. Erasmus published in the Journal of Microscopy, vol. 130 (1983), pp. 187-201 and by S. J. Erasmus and K. C. A. Smith published in the Journal of Microscopy, vol. 127 (1982) pp. 185-199. The method described therein monitors the contrast of the image of a thin film of an amorphous material while the defocus, astigmatism (in x and y directions) and the beam tilt (in x and y directions) are being varied. It utilizes the fact that there is a global minimum in the contrast of a thin amorphous material imaged by a high resolution TEM when the five parameters are varied, and that this minimum corresponds to a well defined condition of a correctly stigmated and aligned microscope set to the Gaussian (minimum phase contrast) defocus. The method has been demonstrated to be able to autoadjust an electron microscope with the required precision. However, in order to find the global minimum, it needs to acquire and analyze about 20 images while each of the 5 parameters is varied. Thus the method needs a total of about 100 images per one pass. This makes it very slow, and hence unsuitable for the daily operation of a TEM. The method also typically results in an unacceptably high irradiation dose delivered to the sample while the microscope is being adjusted. Moreover, the analysis of 100 images requires a large amount of computational power, making special image-processing hardware such as an array processor essential for the method.
A different method for autoadjusting a TEM was disclosed in U.S. Pat. No. 4,618,766 by K. D. van der Mast and U. Gross. The method described therein varies the angle of the electron illumination incident on the sample, determines the image shift accompanying the change in the illumination angle, and uses the image shift value to find the optimum setting of the microscope. This method needs only about 10 images to autoadjust a TEM, and works well at low magnifications. At high magnifications, however, the precision of the method is limited by the fact that the image detail changes when the illumination direction is changed, making it impossible to determine the image shift with the required accuracy. This makes the method unsuitable when high magnifications are needed, for instance when the atomic structure of the sample needs to be resolved. Further, the computational requirements for determining the image shifts are quite considerable, thus also necessitating the use of special image-processing hardware.
Most users of TEMs would find significant advantage in a method and an apparatus for autoadjusting a high resolution transmission electron microscope operating at medium and high magnification, which achieved the required precision using either a single or very few recorded images, and which performed the required adjustments in only a few seconds. Moreover, the users would find significant advantage in an autoadjustment method whose computational demands could be satisfied by a high-end personal computer not equipped with any additional image-processing hardware.